Semiconductor lasers and Light Emitting Diodes (LEDs) commonly are used to generate optical signals for optical communications systems. These optical signals are transmitted through optical fibers that act as waveguides to transport the signals to their destination. The semiconductor lasers or LEDs typically are packaged in a windowed or lensed Transistor Outline (TO) package (or can). This TO package provides the environmental protection needed by the semiconductor light-emitting devices. Subsequently, this first-level package (the TO can with the device) is aligned to an optical element and a fiber to complete the assembly that will become part of the optical communication system. The aligned subsystem, including a first level package, an optical element, and some provision for an alignment of the fiber, commonly is referred to as an Optical SubAssembly (OSA).
There are two topologically distinct types of light emitting devices. Light emitting devices of the first type include devices that emit the light from a top surface, such as Vertical Cavity Surface Emitting Lasers (VCSELs) and most LEDs. Light emitting devices of the second type emit light from a side surface; this group includes Fabry-Perot lasers(FP), Distributed Feedback (DFB) lasers and Distributed Bragg Reflector (DBR) lasers. The devices that emit light from the top surface commonly are referred to as surface emitting devices and the devices that emit from the side surface commonly are referred to as edge emitting devices.
Optical communication systems require higher densities of fibers and devices in order to transmit more data. Typically, the need for increased density is satisfied by providing Parallel Optics Modules that contain an array of semiconductor lasers that transmits light through an array of lenses and into an array of optical fibers.